Offset printing machines are well known in the art and include a master or plate cylinder having a plate for carrying a transferable image thereon and an impression cylinder for carrying paper to receive the image. A blanket cylinder is interposed between the master cylinder and the impression cylinder such that the blanket cylinder moves eccentrically about a first pivot point to make pressure contact with the master cylinder for receiving the image and subsequently moves eccentrically about a second pivot point for making pressure contact with the impression cylinder to transfer the image to the paper. Such system is disclosed in U.S. Pat. No. 4,691,631.
Two-color offset printing machines are also known and they include a single impression cylinder, two master cylinders and two blanket cylinders. A first blanket cylinder is caused to move eccentrically about a first axis to make contact with a first master cylinder and receive the image therefrom and then move eccentrically about a second pivot point to make contact with the impression cylinder and transfer the first color to the paper on the impression cylinder. The impression cylinder then rotates and carries the image on the paper to the other pair of master and blanket cylinders where the second blanket cylinder moves eccentrically into contact with the second master cylinder to receive the second color and subsequently moves eccentrically into contact with the impression cylinder to transfer the second color to the paper. This system is also disclosed in U.S. Pat. No. 4,691,631.
In both the single and double color offset printing machines, a predetermined contact pressure must be maintained between the blanket cylinder and the master cylinder and between the blanket cylinder and the impression cylinder. Thus, the mounts for eccentrically supporting the blanket cylinder are rotationally forced against a fixed but adjustable stop which is manually adjusted in the prior art to allow a desired amount of pressure at the contact points between the master and the blanket cylinders and between the blanket and impression cylinders, respectively. Adjustment is time-consuming, but readjustment is required for wear and the like as the press is generally set up to print with a relatively fixed thickness of plates, blankets and paper stock. Adjustment, if it is required, is accomplished by varying the thickness of the packing sheet underneath the plate on the master cylinder or underneath the blanket on the blanket cylinder or by varying the center distance of the cylinders.
Small sheet-fed presses, in particular, are required to accommodate a wide range of plate and paper stock thicknesses so such semipermanent adjustments mentioned above are unusable. If the settings are to be made frequently, then they must be done simply and quickly, which is difficult, as the adjustment of the actuating means requires that the stops on each end of the cylinders must also be accurately readjusted. Because it is necessary for the eccentrics to rotate freely, thereby necessitating some clearance or springiness as in the case of bearings, if rigid stops, such as used in adjusting a web-type press, are not used to rotationally position the eccentrics, there will be a looseness which will allow the printing cylinders to bounce slightly which will be visible in the printing. Wear will obviously increase the looseness. This bouncing of the cylinders is caused by the manner in which they operate. Levers comprise part of a toggle for selectively rotating a first eccentric and an eccentric shaft such that the first eccentric moves against stops which arrest its rotation, thus providing a predetermined gap clearance between the blanket cylinder and the master cylinder and therefore providing a predetermined contact pressure. For a significant portion of the revolution of the blanket cylinder and the master cylinder, there is no impression or contact pressure due to gaps for plate and blanket lock-ups and during this time the cylinders tend to move together as there is no resisting force. The cylinders rotate around to where the printing begins again and impact occurs which tends to push the cylinders apart. They are eventually restored to their former positions after one or more reverberations. This ringing of the printing cylinders occurs and is disruptive of proper printing.
Additionally, when this occurs between, for instance, the master cylinder and the blanket cylinder, the effect will be felt at the image transfer point between the blanket cylinder and the impression cylinder. It is an effect commonly referred to as streaking or slurring. It is a particular problem when the master cylinder gap and the blanket cylinder gap do not come together the same instant as the blanket cylinder and the impression cylinders since, in this case, the effect of the gap impact occurs somewhere in the middle of the page where it is likely to be noticeable.
Although it is not possible to totally eliminate this effect, it may be minimized to an acceptable level if the blanket cylinder throw-off controls are designed to be very rigid and the forces which restrain the eccentrics are caused to be directed in essentially the same direction as the impression forces and to be substantially greater in magnitude. These forces consist of the force at the toggle that causes the first eccentric movement of the blanket cylinder and the resisting force from the stop mechanism. The vector sum of these two forces must be greater than, and directed between (i) the force vector of the contact between the master cylinder and the blanket cylinder and (ii) the force vector between the blanket cylinder and the impression cylinder. In such case, even though a certain amount of looseness is required in order that the eccentrics be free to turn, the eccentrics are pressed against their mounts in essentially the same direction as the impression pressures and any tendency for the eccentrics to move within their mounts is virtually eliminated. Thus, it is seen that the adjustment of the eccentric actuating means and the adjustment of the stop require a balancing of forces. If the stop force is too great, the toggle lever cannot be locked and, if it is too little, the eccentric actuating means will be loose and there will be a possibility of streaking and slurring of the printed image. On presses where impression pressure adjustments are seldom made, as in the web-type press, this manual adjustment can readily be accommodated. However, on presses requiring frequent changes in paper and plate thickness, protracted adjustment procedures are undesirable.
In the prior art, the structure used to overcome these problems consists of an eccentric shaft supported on both the drive and operator sides by eccentric sleeves. The sleeves coact, but are independently adjusted, and require readjustment from time to time.
The improved design herein eliminates the need for sleeve adjustment on each side and eliminates a significant part of the mechanism on the non-operator side.
In the present invention, an eccentric tube is mounted between the frame side walls of the printing mechanism and is rotated to cause the blanket cylinder to move eccentrically into and out of pressure contact with the master cylinder. An eccentric shaft is inserted within the eccentric tube and is mounted at each end in the frame's side walls and is rotated to cause the blanket cylinder to move eccentrically into and out of pressure contact with the impression cylinder. Pneumatic cylinders are connected to levers which comprise part of a toggle for selectively rotating the eccentric tube and the eccentric shaft. The eccentric tube moves against a stop which arrests its rotation, thus providing an exact gap between the blanket cylinder and the master cylinder and causing an exact impression pressure. Because the tube is eccentrically mounted on the eccentric shaft, rotation of the eccentric tube changes the position of its pivot point which changes the amount of movement of the blanket cylinder into and away from the master cylinder. In this manner, the contact pressure is adjusted as desired in a simple and efficient manner.
In addition, an eccentric cam is used for a stop means and is rotatable about its pivot point so that the amount of movement of the blanket cylinder towards and away from the impression cylinder is adjustable to maintain a constant or predetermined contact pressure in the same general direction as the impression forces.
It is a major object of the present invention to provide an offset printing machine with an eccentric tube supported by an eccentric shaft to enable independent and sequential adjustments and throw-offs for both the plate/blanket cylinders and the blanket/impression cylinders.
Thus, it is another object of the present invention to use the force from an air cylinder acting through linkages to produce a force on the eccentric tube which is in the same general direction as the resultant of the image transfer and the impression forces.
It is also an object of the present invention to use a stop arm formed as a curved extension acting on an eccentric wheel or a cam to limit the travel of the linkage and thus the relative rotary position of the eccentric tube with respect to the eccentric shaft. Such operation provides for adjustment by the operator while at the same time providing additional force to supplement that of the air cylinder to provide the "external force" to prevent rotation of the eccentric shaft within the eccentric tube.
It is a further object of the present invention to eliminate the need for eccentric sleeve adjustment on each side of the offset printing machine and simplify the adjustment mechanism on the non-operator side.
In a second embodiment, a substantially L-shaped lever is attached to the eccentric shaft for movement of the eccentric shaft to adjust the impression cylinder/blanket cylinder contact pressure and throw-off. A teardrop shaped lever is attached in the large portion thereof to the eccentric tube member to adjust the plate cylinder/blanket cylinder contact pressure and throw-off. A triangular shaped lever is pivotally attached to the outer end of the short leg of the L-shaped lever at a first pivot point in one apex of the triangle. Another pivot point in another corner of the triangular shaped lever is superimposed over the axis of the eccentric shaft and is pivotally attached to the outer end of a piston of an air driven cylinder. The third pivot point in the third apex of the triangular shaped lever is coupled to a link that connects to a pivot point in the small outer end of the teardrop shaped link or lever coupled to the eccentric tube member. With this arrangement, adjustment of the blanket/plate cylinder pressure and throw-off can be made substantially without affecting the adjustment of the blanket/impression cylinder pressure. However, when the blanket/impression cylinder adjustment and throw-off is made, both the eccentric shaft and the eccentric tube move in unison.
In still another embodiment, a lever is secured to the eccentric shaft and has three pivot points spaced with respect to each other to form an acute triangle. Again, a teardrop shaped lever is coupled at its large end to the eccentric tube member. A first link has one end pivotally coupled to the small outer end of the teardrop shaped lever and the other link has one end coupled to the upper pivot point of the lever coupled to the eccentric shaft. The other ends of both of the links are pivotally coupled to the outer end of the piston of a first air cylinder such that movement of the air cylinder piston moves both the eccentric shaft and the eccentric tube member simultaneously. One of the lower pivot points of the lever coupled to the eccentric shaft is attached through a linkage to the outer end of a second air cylinder driven piston such that rotation of the eccentric shaft by the second air cylinder does not substantially affect the position of the teardrop shaped lever coupled to the eccentric tube member. Incremental adjustments are available for adjusting both the impression cylinder and the plate cylinder pressure contacts with the blanket cylinder.